A chromosomal gene (fimA) which encodes a subunit of type 2 fimbriae of Actinomyces viscosus T14V has been sequenced. The results revealed an open reading frame of 1605 nucleotides encoding a protein of 56 kDa. The (ATG) initiation codon is preceded by a strong consensus ribosome binding site located 13 nucleotides upstream. A putative leader sequence of 33 amino acids has been identified; hence, the mature subunit (p53) is 53 kDa in size. Translation is terminated by two (TGA) stop condons and a putative (rho-dependent) transcription terminator is located 25 nucleotides further downstream. Analysis of codon usage showed a strong preference for C in the third position. The amino acid composition of the mature subunit as deduced from the nucleotide sequence agreed well with the chemical analysis of affinity-purified type 2 fimbriae. Comparison of the primary amino acid sequence with that of the subunit of A. naeslundii WVU45 (type 2) fimbriae indicated a surprising degree of similarity (77%), especially considering the fact that polyvalent and monoclonal antibodies against these fimbriae barely cross-react. It appears likely that antigenic sites on these fimbriae have diverged rapidly under the immunological pressure of salivary antibodies in the month. A much lower degree of similarity (49%) was noted between type 1 and type 2 fimbriae of A. viscosus T14V.